In most of large-scale networks, a mobility management system by network such as proxy mobile IP (Proxy Mobile Internet Protocol: PMIP; see the Non-Patent Document 1 as given below) is adequately set up. This makes it possible to simplify the arrangement of a user terminal (User Equipment: UE), and operators can perform control more effectively in service behavior. In this type of network, address of the user terminal is not normally changed due to a handover by the user terminal. As a result, mobility can be hidden to an application layer.
On the other hand, as a technique to provide multimedia service by using packet communication network of a cellular phone, IP multimedia subsystem (IMS) is known (see the Non-Patent Document 2 as given below). IMS is a technique, which has been developed to accomplish the Internet Protocol in core network. It is a system, which is useful for integration of voice communication and data communication and to define a system basis to provide various types of services.